This invention relates to methods for determining the thickness of thin films on small areas of structures used in microelectronics fabrication, e.g., near a semiconductor wafer's edge or on a damascene-type structure.
During the fabrication of microelectronic devices, thin films of metals and metal alloys are deposited on silicon wafers and for use as electrical conductors, adhesion-promoting layers, and diffusion barriers. Microprocessors, for example, use metal films of copper, tungsten, and aluminum as electrical conductors and interconnects, titanium and tantalum as adhesion-promoting layers, and titanium:nitride and tantalum:nitride as diffusion barriers. Thickness variations in these films can modify their electrical and mechanical properties, thereby affecting the performance of the microprocessor. The target thickness values of these films vary depending on their function: conductors and interconnects are typically 3000-10000 angstroms thick, while adhesion-promoting and diffusion-barrier layers are typically between 100-500 angstroms thick. The deposition of each of these films must be controlled such that the film's thickness is within a few percent (e.g., 5-100 angstroms, a value roughly equivalent to one or two seconds human fingernail growth) of its target value. Furthermore, the uniformity of the film over the surface of the wafer must be closely controlled in order to assure uniform behavior of the individual microprocessors and, consequently, high manufacturing yields. Because of these rigid tolerances, film thickness is often measured as a quality-control parameter during and/or after the microprocessor's fabrication.
The metal films are often deposited and patterned in complex geometries and this complicates the measurement process. In a typical fabrication process, a titanium:nitride film is deposited over the entire surface of a silicon wafer. A tungsten film is then deposited onto the titanium:nitride film to leave an "edge-exclusion zone", i.e., a small (about 1 or 2 mm) region where the titanium:nitride is exposed, near the wafer's edge. The edge-exclusion zone prevents delamination of the tungsten film near its edges. Near this region, the thickness of the tungsten film rapidly increases to its target value; this takes place over a distance of a few hundred microns. Without this rapid increase in film thickness, devices patterned near the wafer's edge-exclusion zone will contain non-ideal tungsten films not having adequate thickness, and they will not meet specifications.
An example of a complicated film geometry recently introduced in commercial microelectronics fabrication is a "damascene" or "dual damascene" structure. These structures, used especially to form copper conductors and interconnects, are typically formed by a multi-step process: copper is first deposited onto a wafer having a dielectric layer that has been etched to have a series of trenches; the wafer is then polished by chemical-mechanical polishing (CMP) to remove excess copper, leaving only copper-filled trenches. The resulting structure is typically a series of separated copper lines having a thickness of a few thousand angstroms, a width of about 0.5 microns, a periodicity of about 2 microns, and a length of several millimeters.
Measuring film thickness in and near the edge-exclusion zone and in damascene-type structures is difficult and impractical using conventional techniques. For example, blanket metal films are typically measured using a 4-point probe. Here, two separated pair of conducting probes contact the film; electrical resistance, as measured by the probes, relates to the film's thickness. Because the spatial resolution of the 4-point probe is typically a few hundred millimeters, this instrument is impractical for both edge-profile and damascene-type structures. Moreover, a film's resistance often depends on both its thickness and geometry, a complication that further reduces the accuracy of the 4-point probe when used to measure complex geometries. Another film-measuring instrument, called a stylus profilometer, drags a stylus needle over a sample, recording variations in topography. This instrument, however, is slow, cumbersome, sensitive to slight amounts of sample curvature, and inaccurate when used to measure relatively long distances (e.g., the hundreds of microns required for tungsten build-up near the exclusion zone).
In addition to these disadvantages, both 4-point probes and stylus profilometers require contacting and thus contaminating a sample. These instruments are therefore typically used on monitor or test samples, rather than samples containing actual product. Other methods for measuring the thickness of metal films, such as X-ray fluorescence and Rutherford backscattering, are non-contact, but are slow and have poor spatial resolution.